Migratory insertions significance

In situ NMR analysis has also been used to determine the kinetic barriers for the migratory insertions of methyl carhonyl complexes [Pd CO) Me)(PPh2 CH2) PPh2)] (n = 2-4) relevant to propagation in ethene/CO copolymerisation. It was found that the steric bulk of the diphosphine has a significant effect on the insertion barriers with the most bulky ligand having the lowest barrier. 

The fact that there is such a paucity of metal formyl complexes is both interesting and significant because of the proposed intermediacy of coordinated formyl in CO reduction, and the sharply contrasting abundance of metal acyl complexes. Since many of the acyl complexes are known to form by migratory insertion of CO in an alkyl carbonyl complex (20, 20a, 22), the lack of formyl complexes from hydride carbonyls relates to the thermodynamic difference in the equilibrium (5) when Y is alkyl and when it is hydride. 

A parallel situation appears to obtain for the mixed allyl nitrosyl complex Ru(NO)(C3H5)L2 prepared by Schoonover and Eisenberg (231). This complex which is coordinatively saturated (NO+ and rf -allyl), forms a CO adduct which is assigned a bent nitrosyl structure (231). Further reaction under CO leads to the formation of Ru(CO)3L2 with the possible elimination of acrolein oxime. The coupling of the allyl and nitrosyl ligands can be viewed in this case as nucleophilic attack of NO- on an f/3-allyl species. Unlike in reaction (110), both of the moieties to be coupled lie within the same coordination sphere. The significance of these results is that it lends viability to the notion embodied in (109) in which a migratory insertion of nitrosyl occurs as NO-. 

While the effect of the hydroxylic solvent on k2 cannot be quantified, it would be expected to accelerate it. The work of Hickey and Maitlis (19) involved the use of nonhydroxylic solvents, in which this accelerating effect is not available, and the assumption that k2 is fast under these conditions may not be justified. It is even possible that it may be slowed to the extent that it contributes to the rate-limiting step. If this is the case, the rate enhancements observed in their work might also be attributable to the acceleration of the k2 step by the Lewis acid countercations. Such effects of cations on migratory insertions are expected to be more important under their experimental conditions, where ion pairing was acknowledged to be significant. However, while this alternative explanation may appear to be consistent with the available data, we must emphasize that it is speculative. Certainly, more detailed studies are required to determine definitively the reason for this acceleratory effect. 

Also noteworthy are some alkylidenes that exemplify rare reactivity for metal hydrides. The first is the cyclic carbene complex 565, the formation of which is itself unusual, proceeding as it does from the interaction of Bp Rh(CO)(py) (566) and methyl iodide. This is proposed to involve the oxidative addition of Mel and subsequent migratory insertion of CO, though at what stage the B-H activation occurs remains to be determined. More significant, however, is that on heating to 45 °C, 565 irreversibly evolves into the alkyl complex 567 via a rare reverse a-hydride migration onto the alkylidene carbon (Scheme 55, Section II-D.2). 

Elementary steps in binuclear catalysis can differ significantly from those described for mononuclear complexes due to the proximity of a secrmd metal center. A brief description of binuclear oxidative addition, reductive elimination, ligand migration, and migratory insertion will be made in order to facilitate the understanding of the mechanisms discussed in this chapter. 

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